A cornerstone of several common therapies for human diseases is the surgical use of a vein as an arterial conduit. Autologous vein bypass grafts are commonly performed for advanced cardiovascular and peripheral vascular disease; similarly, surgeons also perform arteriovenous fistulae (AVF), the preferred access for hemodialysis. The poor patency of both vein grafts and AVF, requiring additional re-do procedures and surgery, reflects our imperfect understanding of the biology of venous remodeling that leads to successful venous adaptation to the arterial environment. This knowledge gap creates an unmet medical need for novel approaches to enhance venous adaptation, maturation, and successful long-term use of venous conduits. The tyrosine kinase receptor Eph-B4 is an embryonic determinant of veins. Diminished Eph-B4 expression is associated with vein graft adaptation in humans and mice, and reduced Eph-B4 activity is essential for venous adaptation in adult mouse vein grafts. We present exciting new data that: 1) our innovative mouse model of AVF faithfully recapitulates human AVF maturation, including a subset that fail to mature; 2) in both humans and mice, Eph-B4 expression increases during AVF adaptation to the arterial environment, unlike the decreased Eph-B4 expression during vein graft adaptation; 3) Eph-B4 function is essential for AVF adaptation; and 4) Eph-B4 tyrosine-774 is a critical site of Eph-B4 phosphorylation and downstream signaling. We hypothesize that vein graft adaptation leads to loss of vessel identity and AVF maturation leads to merged arterial-venous identity. Our data also suggest a mechanism, e.g. that Eph-B4 is a previously unrecognized but critical endothelial transducer of hemodynamic loads to veins. We hypothesize that differences in the hemodynamic environments of vein grafts and AVF differentially regulate Eph-B4 tyrosine-774 phosphorylation and/or Eph-B4 expression. We will use a combination of in vitro and in vivo models, using a bioreactor that can individually control separate hemodynamic loads on both large and small vessels and endothelial monolayers, as well as using in vivo models of vein grafts and AVF, to test our hypothesis with the following specific aims: Aim I: Determine how hemodynamic loads differentially regulate Eph-B4 phosphorylation and function in whole vessels, including analysis of successful and failed AVF. Aim II: Determine how different magnitudes of shear stress regulate Eph-B4 phosphorylation and function in endothelial cells. Aim III: Determine how Eph-B4 tyrosine-774 phosphorylation regulates venous endothelial cell function. The work in this proposal will establish our innovative and paradigm-changing hypothesis that Eph-B4 is a novel and critical transducer of mechanical loads to the blood vessel and that Eph-B4 is differentially responsive to different types of loads, even in the absence of Ephrin-B2. Eph-B4 activity or lack thereof, defines the phenotype of the blood vessel and its function. Abnormal regulation of vessel identity during vein graft adaptation and AVF maturation can be manipulated to improve human clinical therapies.